Assess Impacts Of Feral Horses and Livestock on Sage-Grouse Demography and Habitat

Sage-grouse were determined by U.S. Fish and Wildlife Service (USFWS) to be warranted for listing under the Endangered Species Act but listing was precluded by other higher-priority species. USFWS has been ordered by a federal district court judge to make a final determination by 2015. Numerous factors, including energy development, loss of habitat to agriculture (especially in Washington and southern Idaho), wildfire, diseases and livestock grazing, have been implicated in the decline.

Potential impacts of livestock grazing are, however, confounded with the grazing impacts of feral horses, especially in the western part of the sage-grouse distribution. Ranchers have complained that they are unfairly blamed for, what are in fact, impacts caused by horses. Additionally, advocates for feral horses have resisted appropriate management of the abundance of horses on public lands.

The Bureau of Land Management now spends about $75M annually on management of feral horses on their lands. It is therefore important to better understand the relative impacts of feral horses versus livestock on vegetation and the associated impacts on sage-grouse populations.

This project takes advantage of a unique combination of historical data and current distributions of feral horses and livestock to separate the effects of horses and livestock on Great Basin vegetation and sage-grouse.

Greater Sage-grouse (Centrocercus urophasianus) an iconic species of the Great Basin were recently determined to be warranted for listing under the Endangered Species Act by the U.S. Fish and Wildlife Service. An important contributor to declines in sage-grouse populations has been degradation of key habitats associated with energy development, urbanization, climate change. Grazing by livestock and feral horses have also been implicated, although livestock producers have pointed out that much of the impact of grazing in the northern and western Great Basin is associated with feral horses.

Grazing reduces cover for nests and food for breeding adults and broods. Both effects are important because vegetative cover affects nest success and food affects brood survival. Both recruitment and adult survival are important to local population dynamics in sage-grouse populations.

Feral horses are currently substantially affecting vegetation on Sheldon National Wildlife Refuge (Collins 2011). Other scientists have demonstrated substantial impacts of feral horses on grasses, forbs and shrubs. It has generally not been possible to separate the effects of feral horses from those of livestock on Great Basin plant communities and associated wildlife because both types of grazing typically occur together.

The Sheldon-Hart Refuge Complex and adjacent federal lands offer a unique opportunity to assess the impacts of feral horses in the absence of livestock grazing because livestock were removed from the refuge in the early1990s and horses have increased substantially on the Sheldon but not on Hart Mountain over the last decade. Lands adjacent to the Sheldon National Wildlife Refuge have had continuous grazing by both horses and cattle, and, therefore provide a control for reductions in grazing on the Refuge units.

Consequently, it is possible to compare vegetation and sage-grouse on the two units of the Refuge and the Vya-Massacre sage-grouse population management units (PMUs) on land immediately southwest of the Sheldon unit; the Sheldon unit has substantial horse impacts, the Hart Mountain unit supports only low densities of horses, and the adjacent lands have both cattle and horses.

Grazing by feral horses is expected to have the greatest impact on recruitment of young because reduced understory vegetation could negatively impact nest success and survival of young. Potential effects on adult survival are unknown because few estimates of survival exist for sage-grouse, making it difficult to understand ecological processes that might impact survival. Two different studies have implicated harsh weather during winter in increased mortality and three other studies have detected increased mortality during the fall period. Increased fall mortality represents a complex interaction between predation and availability of food.

Evaluation of cattle and horse impacts is substantially strengthened in this case because Dr. Mike Gregg collected substantial historical data on both vegetation and sage-grouse from both the Hart Mountain and Sheldon units over the period 1988-2005, which will be combined with data to be collected during the current study. This project repeats Dr. Gregg’s work on the Hart and Sheldon units of the Refuge and on land adjacent to the Refuge. Work on non-refuge land is a high priority for the Nevada Department of Wildlife and the Surprise Field Office of BLM. Combining work on the Vya-Massacre PMUs with that on the Hart-Sheldon Refuge has the advantage that data can be brought into a single analysis that will provide the greatest power to assess grazing impacts on vegetation and sage-grouse.

A combined study provides an opportunity to separate the effects of cattle grazing from that of horses, and effects of both non-native ungulates from habitats that are ungrazed.

The main objective of this research is to separate effects on Great Basin vegetation and sage-grouse of feral horses from those of livestock.

Specific objectives to meet the main objective are:

Measure understory and shrub vegetation in relation to use by horses or livestock.

Estimate breeding propensity, nest success, chick survival, adult survival and habitat use by sage-grouse in relation to vegetation variables and habitat use by horses or livestock.

Estimate population trends of sage-grouse in relation to vegetation variables and use by horses or livestock.